1. Field of the Invention
The present invention relates to apparatus and methods for controlling tension in webs.
2. State of the Art
In recent years, workers in the battery art have begun to understand and recognize the advantages of so-called laminate batteries that include solid polymer electrolytes and sheet-like anodes and cathodes. The advantages of such batteries include lower battery weights than conventional batteries that employ liquid electrolytes, longer service life, and relatively high power densities. The advantages of laminate batteries also include relatively high specific energies, and the elimination of the danger associated with batteries containing spillable liquid electrolytes such as acids.
A laminate battery is disclosed in U.S. Pat. No. 4,925,751. According to that patent, a cathode material is formed from a mixture of an active cathodic material (preferably vanadium oxide V.sub.3 O.sub.8 or V.sub.6 O.sub.13), a conductive filler material (preferably carbon particles or filaments), and an ionically conductive polymer electrolyte material is laminated on a conductive substrate material such as a nickel or copper web or sheet. A layer of polymer electrolyte material is laminated over the laminated layer of cathode material, and an anode material is applied over the laminated layer of polymer electrolyte material.
The anode material is preferably in the form of a thin sheet or web of lithium foil. Recent efforts to mass produce polymer electrolyte batteries have involved the use of a lithium foil web that is approximately 2 mils thick and approximately 13/4" wide and is, accordingly, quite delicate. The anode web is applied to the electrolyte layer of the electrolyte and cathode laminate by suitable means such as a calendaring device.
The anode web is generally unwound from a roll of anode web material and is then applied to the electrolyte and cathode laminate. The anode, electrolyte, and cathode laminate formed in this manner is generally either rewound onto another roll or cut or otherwise processed to form a desired battery configuration. As the anode material is unwound from an unwinding roller, to maintain a constant speed of the web, it is necessary to increase the rotational speed of the unwinding roller to compensate for the decreasing diameter of the roll of anode material. Similarly, it is also generally necessary to, for example, decrease the rotational speed of any wind-up roller onto which an increasing diameter anode, electrolyte, and cathode laminate is wound.
Changes in speeds of the unwinding and wind-up rollers generally result in changes in tension of the anode web. Accordingly, devices such as tension control rollers have been used to maintain a desired level of tension in the web. Such devices, however, involve direct contact with a surface of the delicate anode web.
When the anode material is applied over the polymer electrolyte and cathode material laminate, it is necessary to avoid tears and similar imperfections in the foil web to provide proper battery performance and avoid failures due to shorting. Further, it is desirable to maintain a substantially constant tension in the anode web during its application to the electrolyte and cathode laminate to avoid wrinkles in or breakage of the anode web. Because of the delicacy of the anode web, conventional tensioning devices have not performed as well as desired and contact between the web and the tensioning devices tends to damage the web, such as by causing tears, wrinkles, and the like, resulting in inconsistent quality.
It is, accordingly, desirable to provide a device adapted to tension a delicate web. It is further desirable to provide a tensioning device that requires minimal direct physical contact with the web.